1. Field of the Invention
This invention relates to electrostatic shields for electrical transformers, and in particular relates to an improvement in the dielectric strength thereof.
2. Description of the Prior Art
Typically, electrical transformers used in air or in an insulating gas comprise a cylindrically wound coil with insulating spacers disposed between the windings thereof, and an electrostatic shield disposed at each of the two ends of the coil. Heretofore these electrostatic shields were formed in a roughly "doughnut" shaped ring with an asymmetric cross-section, with one flat and one curved side. In cross-section, this "doughnut" ring comprised an inner insulator, around which was wound a field concentration relaxation conductor formed from a conductive foil, and wound around this relaxation conductor was a polyethylene terephthalate film (hereinbelow referred to as PET film), which formed an outer insulator.
When a voltage was applied to a transformer thus constructed, a field of differing local strengths and directions was produced, with a field of great strength produced in the electrostatic shields. The construction of the electrostatic shields involved the problem, however, that their dielectric for a transformer strength was not very high, and dielectric breakdowns occurred between the field concentration relaxation conductor and ground through the PET film, particularly at certain points where field strength was greatest.
In order to increase the dielectric strength of the electrostatic shields, both increasing the number of windings of the PET film, to increase the thickness of the outer layer, and providing the electrostatic shields with a greater degree of curvature have been considered, but neither method is able to achieve more than a small increase in dielectric breakdown voltage, and these methods also involved increased physical dimensions of the electrostatic shields, and significantly increased costs.